I have found it to slow down my machine considerably. Not all the time, but there are long pauses at times when everything seems to be running in molases.
Mail seems to be part of it taking longer to do many tasks and inexplicably chewing up cpu usage.
Then there is the memory issue. I have to run Free Memory a LOT more often than before ( great utility - you should all have it) - and this brings the system to a halt.
Perhaps some of the power usage is...